Showing papers by "L. N. Pfeiffer published in 1999"

Tunneling between the Edges of Two Lateral Quantum Hall Systems

Abstract: The edge of a two-dimensional electron system (2DES) in a magnetic field consists of one-dimensional (1D) edge-channels that arise from the confining electric field at the edge of the specimen$^{1-3}$. The crossed electric and magnetic fields, E x B, cause electrons to drift parallel to the sample boundary creating a chiral current that travels along the edge in only one direction. Remarkably, in an ideal 2DES in the quantum Hall regime all current flows along the edge$^{4-6}$. Quantization of the Hall resistance, $R_{xy}= h/Ne^{2}$, arises from occupation of N 1D edge channels, each contributing a conductance of $e^{2}/h^{7-11}$. To explore this unusual one-dimensional property of an otherwise two-dimensional system, we have studied tunneling between the edges of 2DESs in the regime of integer quantum Hall effect (QHE). In the presence of an atomically precise, high-quality tunnel barrier, the resultant interaction between the edge states leads to the formation of new energy gaps and an intriguing dispersion relation for electrons traveling along the barrier. The absence of tunneling features due to the electron spin and the persistence of a conductance peak at zero bias are not consistent with a model of weakly interacting edge states.

Nonmonotonic temperature-dependent resistance in low density 2d hole gases

Abstract: The low temperature longitudinal resistance-per-square Rxx(T) in ungated GaAs/AlGaAs quantum wells of high peak hole mobility 1.7x10^6 cm^2/Vs is metallic for 2D hole density p as low as 3.8x10^9 cm-2. The electronic contribution to the resistance, R_{el}(T), is a nonmonotonic function of T, exhibiting thermal activation, R_{el}(T) ~ exp{-E_a/kT}, for kT EF. The form of R_{el}(T) is independent of density, indicating a fundamental relationship between the low and high T scattering mechanisms in the metallic state.

Effective mass and g factor of four-flux-quanta composite fermions

Abstract: We investigate the properties of composite fermions with four attached flux quanta through tilted-field experiments near Landau level filling factor {nu}=3/4 . The observed collapse of fractional quantum Hall gaps in the vicinity of this quarter-filling state can be comprehensively understood in terms of composite fermions with mass and spin. Remarkably, the effective mass and g factor of these four-flux-quanta composite fermions around {nu}=3/4 are very similar to those of two-flux-quanta composite fermions around {nu}=3/2 . {copyright} {ital 1999} {ital The American Physical Society }

Single-electron capacitance spectroscopy of vertical quantum dots using a single-electron transistor

Abstract: We have incorporated an aluminum single-electron transistor (SET) directly on top of a vertical quantum dot, enabling the use of the SET as an electrometer that is extremely responsive to the motion of charge into and out of the dot. Charge induced on the SET central island from single-electron additions to the dot modulates the SET output, and we describe two methods for demodulation that permit quantitative extraction of the quantum dot capacitance signal. The two methods produce closely similar results for the determined single-electron capacitance peaks.

Quantum point contact in a magnetic field: Far-infrared resonant heating observed in photoconductivity

Abstract: We report on the far-infrared photoresponse of a quantum point contact device fabricated on a top-gated GaAs/AlGaAs heterostructure. The top-gated architecture avoids the disorder built into conventional modulation-doped structures. We observe a distinctive far-infrared magnetophotoresponse. This depends on the wavelength of the radiation and on the carrier density, which is controlled by the gate voltage. We conclude by comparison with transport data that the oscillations observed in photoconductivity and which are centred around the cyclotron energy arise from the resonant heating of electrons by the far-infrared radiation.

Magnetic-field-induced cavity polariton linewidth reduction in a G a A s / A l 0.1 Ga 0.9 As microcavity

Abstract: The magnetic-field $(Bl~7\mathrm{T})$ effect on the cavity polaritons linewidth is studied by reflection spectroscopy at $T=2--5\mathrm{K}.$ The structure under study consists of a single 160-\AA{} GaAs/AlAs quantum well (QW) embedded in a \ensuremath{\lambda}-wide ${\mathrm{Al}}_{0.1}{\mathrm{Ga}}_{0.9}\mathrm{As}$ microcavity (MC) that is cladded by ${\mathrm{Al}}_{0.1}{\mathrm{Ga}}_{0.9}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}\mathrm{A}\mathrm{s}$ distributed Bragg reflectors. The QW absorption above the QW band gap, $EgE(e1\ensuremath{-}\mathrm{hh}1),$ is flat and it transforms into a series of well-defined bands (Landau transitions or magnetoexcitons) under a perpendicularly applied magnetic field. This results in a narrowing of the MC mode by about 30% when ${E}_{\mathrm{MC}}gE(e1\ensuremath{-}\mathrm{hh}1).$ We analyze the reflection spectra by using the linear dispersion model based on the transfer-matrix formalism and introducing the $(e1:\mathrm{hh}1)1S,$ $(e1:\mathrm{lh}1)1S$ excitons and the $N=1$ and $N=2$ interband Landau transitions as Lorentzian oscillators. The model explains well the reduction in the MC mode linewidth, under the applied magnetic field, as due to the change in the absorption spectrum caused by the transformation from a two- to zero-dimensional electronic density of states. It is also found that the MC mode linewidth and intensity, in the spectral range ${E}_{\mathrm{MC}}lE(e1:\mathrm{hh}1)1S,$ are strongly affected by below-band-gap residual absorption of the ${\mathrm{Al}}_{0.1}{\mathrm{Ga}}_{0.9}\mathrm{As}$ layers.

Confined 1 S excitons, Landau transitions, and cavity mode coupling in GaAs microcavities

Abstract: The reflection spectra of GaAs microcavities (MC) formed between ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ distributed Bragg reflectors were studied at $T=2\mathrm{K}$ and under a magnetic field applied perpendicularly to the layers (z direction). Several MC's were studied, whose length ${L}_{\mathrm{MC}}\ensuremath{\sim}{\ensuremath{\lambda}}_{\mathrm{exc}}=223\mathrm{nm}$ corresponds to the $1S$ exciton wavelength of bulk GaAs at $T=2\mathrm{K}$ $[E(1S)=1.515\mathrm{eV}].$ The spectra show a large number of sharp lines either within the broad MC-confined photon band, when the MC mode overlaps the continuum absorption, or within the upper Rabi split band when the MC mode is near resonance with the lowest $1S$ exciton states. The energy and intensity of these sharp lines vary with increasing magnetic field $(0lBl~4\mathrm{T}).$ For comparison, the reflection spectrum of a ${\ensuremath{\lambda}}_{\mathrm{exc}}$-wide GaAs layer (cladded by 100 nm-wide ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ layers) was similarly studied. It did not show the magnetic-field-dependent features observed in the MC samples. It is shown that the reflection spectra of the MC samples are determined by the resonant coupling between three types of excitations: (a) $1S$ excitons with ${k}_{z}g0$ that are spatially confined in the ${\ensuremath{\lambda}}_{\mathrm{exc}}$-wide MC, resulting in a quantization of their center of mass motion. (b) Landau transitions (magnetoexcitons) between electron and hole Landau levels with indices ranging up to $p=11.$ (c) MC-confined photons. The reflection spectra are calculated by constructing transfer matrices that describe light propagation along the z direction (in the MC and in each layer of the distributed Bragg reflectors) and introducing an energy-dependent dielectric function for the GaAs layer, in the form of a Lorentzian oscillator response function for the $1S$ exciton and for each one of the magnetoexcitons. The $1S$ exciton center of mass quantization is introduced by including its ${k}_{z}$ dispersion within the dielectric function and using Pekar's additional boundary condition. The calculated spectra fit reasonably well the experimental ones (in both energy and intensity). The fan diagrams show anticrossings between the spatially confined $1S$ exciton levels and the $pg~1$ magnetoexcitons, as they are tuned by the magnetic field. These anticrossings are observed experimentally only in the MC mode spectral range, as predicted by the model. Using the same model, the electronic excitations energy dependence on magnetic field is calculated for the ${\ensuremath{\lambda}}_{\mathrm{exc}}$-wide GaAs layer confined between the ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ layers (namely without the distributed Bragg reflectors). No anticrossings are obtained. It is thus concluded that the spatially confined $1S$ exciton levels and the $pg~1$ magnetoexcitons are coupled via their interaction with the MC-confined photons.

Ultrafast spectral interferometry of resonant secondary emission from quantum wells: From Rayleigh scattering to coherent emission from biexcitons

Abstract: Recent investigations of secondary emission from quantum well excitons following ultrafast resonant excitation have demonstrated an intricate interplay of coherent Rayleigh scattering and incoherent luminescence1,2,3,4. We have very recently demonstrated5 that it is possible to isolate and time resolve the coherent field associated with the Rayleigh component using ultrafast spectral interferometry6 or Tadpole7, thus, obtaining substantial and new information of the nature of resonant secondary emission.

Ultrafast interferometric investigation of resonant secondary emission from quantum well excitons

Abstract: Summary form only given. We present the first direct investigation of the coherence properties of resonant secondary emission (RSE) from quantum well excitons. Using ultrafast spectral interferometry, or TADPOLE, we measure spectral fringes that are resonantly enhanced at the exciton transitions. The fringe formation is due to the coherent Rayleigh component of the RSE, and allows us to investigate the dynamics of resonant Rayleigh scattering independent from incoherent luminescence. Our observation of high-contrast spectral fringes is in contradiction with current theories of time resolved resonant Rayleigh scattering from excitons and shows for the first time the non-ergodic nature of the RSE.

Ultrafast interfeometric investigation of resonant secondary emission from quantum well excitons

Abstract: Summary form only given. We present the first direct investigation of the coherence properties of resonant secondary emission (RSE) from quantum well excitons. Using ultrafast spectral interferometry, or TADPOLE, we measure spectral fringes that are resonantly enhanced at the exciton transitions. The fringe formation is due to the coherent Rayleigh component of the RSE, and allows us to investigate the dynamics of resonant Rayleigh scattering independent from incoherent luminescence. Our observation of high-contrast spectral fringes is in contradiction with current theories of time resolved resonant Rayleigh scattering from excitons and shows for the first time the non-ergodic nature of the RSE.

Coherent terahertz emission from cavity polaritons in semiconductor microcavities

Abstract: Summary form only given. The appearance of Rabi oscillations in the time domain in semiconductor microcavities (MC) has long been established ever since the first time-resolved reflection measurement from such samples. In these experiments, a short laser pulse excites the Rabi split exciton polariton and then some optical property are measured as a function of time (four wave mixing, reflectivity, etc). In the work we directly measure the electromagnetic emission due to time-dependent Rabi oscillations. This is the first time, to our knowledge, that terahertz (THz) photons emitted, due to a dipole transition between exciton polaritons embedded in a microcavity, have been observed.

Coherent terahertz radiation from Rabi oscillations in a semiconductor microcavity

Abstract: We report on the first observation of coherent terahertz (THz) radiation emitted from an unbiased semiconductor microcaivty at T=4.7K. The MC consists of three 108A/102A GaAs/AlAs placed at the center of a λ cavity. The THz radiation is emitted after a resonant excitation of the two cavity polaritons with a short pulse. We attribute the THz radiation to dipole oscillations at the vacuum Rabi splitting frequency between the photon and the exciton states.